The rate at which an implanted material resorbs or biodegrades within the body can be a major factor in determining its utility as a biomaterial. So called inert materials, such as metals, ceramics and plastics have been shown to be useful for permanent implants. However, in applications in which a device serves as an aid to healing or as a temporary aid in surgical repair, a resorbable material has the advantage of not having to be removed, once healing has occurred. Resorbable sutures and staples, bone pins and screws, wound dressings, and injectable drug delivery systems or depots are examples of such devices. There are very few materials available today which have the physical, chemical and biological properties necessary for the fabrication of medical devices, which must degrade and resorb in the body without detrimental consequences.
Various synthetic organic polymers have found use, such as polylactides, polyglycolides, polyanhydrides and polyorthoesters, which degrade in the body by hydrolysis. Collagen, glycosaminoglycans and hyaluronic acid are examples of natural implantable materials which resorb at least partially by enzymatic degradation. The rates of resorption are limited to the nature of the particular material and modifications can change the rate of resorption, but at the same time may adversely affect the desired properties of the product.
Illustrative of efforts to vary resorption characteristics by compositional changes are synthetic resorbable sutures composed of copolymers of lactide and glycolide. By varying the ratio of lactic acid to glycolic acid, the rate of resorption may be varied. Unfortunately, rapidly resorbing compositions tend to be soft and weak. Slow resorbing compositions are stiff and strong. However, their resorption, which is hydrolytic, produces acid buffered by the tissue medium, where erosion occurs at the polymer surface. In addition, however, hydrolysis may occur internally, where the resulting acid catalyzes and accelerates the degradation of the polymer. Thus, internal pockets of degradation can lead to rapid and catastrophic failure of mechanical properties.
There is, therefore, a need for products which can be used in the production of implantable devices. Such products should have the desired mechanical properties of tensile strength, elasticity, formability, and the like, provide for controlled resorption, and be physiologically acceptable.
Relevant Literature
U.S. Pat. No. 5,243,038 describes the preparation of high molecular weight, protein polymers and copolymers comprising long segments of small repeating units. Bioactive Polymeric Systems, Gebelein, C. G. and Carraher, C. E., eds., Plenum Press, New York, 1985; Contemporary Biomaterials, Boretos, John W. and Eden, Murray, eds., Noyes Publications, New Jersey, 1984; and Concise Guide to Biomedical Polymers: Their Design, Fabrication and Molding, Boretos, John W., Thomas pub., Illinois, 1973, describe compositions, characteristics, and applications of biomaterials.